1. Field of the Invention
The present invention relates to an optical waveguide-integrated substrate for optical communication, a method for producing the substrate, and an optical transceiver using the substrate.
2. Description of the Background Art
Recent years have seen the development of communication technology relying on optical fibers, and researchers and engineers have developed low-cost, compact optical transceivers. For example, Ryuta Takahashi et al have developed an optical transceiver produced by combining two optical fibers, a laser diode (hereinafter referred to as LD) for signal transmission, and a photodiode (hereinafter referred to as PD) for signal reception on a single-crystal Si substrate having V-shaped grooves. The optical transceiver has been disclosed in a paper entitled “Packaging of optical semiconductor chips for SFF (small form factor) optical transceiver,” which is included in the “Proceedings of the 1999 Electronics Society Conference of IEICE (The Institute of Electronics, Information and Communication Engineers of Japan),” Vol. 1, number C-3-28, page 133.
On the other hand, optical waveguides have been used for the miniaturization and cost reduction of optical transceivers. An example of such optical waveguides has been disclosed in the published Japanese patent application Tokukaihei 11-68705. According to the application, a combination of an optical waveguide and a multi-layer filter achieves a dual function of the signal transmission by a light wave having a wavelength of 1.3 μm and the signal reception by a light wave having a wavelength of 1.55 μm. This function is achieved by forming an optical waveguide on a single-crystal Si substrate to integrate an LD and a PD.
As explained above, it is essential to use an optical waveguide for integrating the optical-signal-transmitting and -receiving devices for an optical transceiver. Consequently, based on the foregoing two prior arts, a structure shown in FIGS. 6(a) and 6(b) can be conceived, for example, for obtaining a high-performance optical transceiver by using an optical waveguide. FIG. 6(a) is a plan view of the optical transceiver, and FIG. 6(b) is a side view showing the cross section along the line A-B shown in FIG. 6(a). The optical transceiver using a singlecrystal Si substrate is explained below. An SiO2-family optical waveguide is formed on a single-crystal Si substrate 101. The optical waveguide comprises cores 103 and 104 for guiding light waves and a cladding layer 102 enclosing the cores 103 and 104. The cladding layer 102 is made of SiO2, and the cores 103 and 104 are made of SiO2 doped with GeO2 and have a refractive index higher than that of the cladding layer 102. A substrate on which an optical waveguide is formed as explained above is referred to as an optical waveguide-integrated substrate 109.
The optical waveguide-integrated substrate 109 has an area 101-1 where no optical waveguide is formed. An LD 107 and a PD 108 are mounted on the area 101-1 for performing signal transmission and reception. To transmit optical signals, the LD 107 is driven by the transmitting signals to emit light waves, which in turn are sent into an optical fiber 105 through the core 103. To receive optical signals, incoming light waves having emerged from an optical fiber 106 enter the PD 108 through the core 104 to be converted into electrical signals. Although not shown in FIGS. 6(a) and 6(b), the signal-transmitting and -receiving functions can be augmented, for example, by forming a Bragg diffraction grating, which reflects a light wave having a specific wavelength, at some midpoint in the core 103 to select a transmitting wavelength as required or by inserting a wavelength-selecting filter, which selectively transmits a light wave having a specific wavelength, at some midpoint in the core 104 to obtain wavelength selectivity in the signal reception.